1. Field of the Invention
The invention relates to an X-ray examination apparatus including an image sensor matrix for picking up an X-ray image, and a correction unit for correcting disturbances in the primary image signal.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from European Patent Application EP 0 642 264, which corresponds to U.S. Pat. No. 5,530,238.
The known X-ray examination apparatus includes a correction unit for counteracting disturbances in the electronic image signal which are caused by delayed reading of charge. Incident X-rays release charge carriers, notably photoelectrons in the image sensor matrix and the X-ray image is thus picked up. A part of said photoelectrons can be intercepted in a trap state, be retained therein for some time, and can escape at a later stage, for example due to thermal excitation, and enter the reading circuit as electric charges read with a delay. If the primary image signal with the disturbances were applied to a monitor for displaying the image information, not only the image information of the instantaneous image would be reproduced, but at the same time also image information of a previously picked up image. As a result, the observer will perceive after-images superposed on the instantaneous image. The correction unit of the known X-ray examination apparatus utilizes a physical model for the interception and subsequent escape of the photoelectrons so as to correct disturbances due to electric charges read in delayed fashion. It is a drawback that the known X-ray examination apparatus requires a rather large image memory capacity for storing image information of a number of preceding images and/or for storing correction values for a long series of X-ray images as well as for different circumstances in which such X-ray images are formed. It is a further drawback that the correction unit of the known X-ray examination apparatus requires a powerful arithmetic unit for executing the desired correction on the basis of the image information stored. Because elaborate and complex operations are performed on a large number of data in the known X-ray examination apparatus, a rather long period of time is required to correct the primary image signal in respect of after-images. The long processing time prolongs the time elapsing between the picking up of the X-ray image and the reproduction of the image information in the X-ray image, for example on a monitor. Therefore, the known X-ray examination apparatus is not suitable for non-disturbed reproduction of the image information in an X-ray image quickly after it has been picked up. This concerns notably disturbances due to the superposition of image information of a previously picked up X-ray image on the image information to be reproduced, said superposed information being reproduced as an after-image together with the image to be reproduced. Consequently, the known X-ray examination apparatus is notably not suitable for reproducing a series of non-disturbed X-ray images in rapid succession.